Bacteria and Yeast Diversity in Flower Nectar Can Increase with Greater Dispersal

MIFUYU NAKAJIMA / COURTESY

Research from UC Davis scientist challenges a traditional ecology theory

A recent study published in Ecology Letters and led in part by UC Davis entomology and nematology professor Rachel Vanette could change scientists’ understanding of how microbes are dispersed in nature.

Mimulus aurantiacus, commonly called Sticky Monkeyflower, is a flowering plant found along the West Coast of the United States. The flower is bright in appearance and can be seen growing and blooming in the UC Davis Arboretum.

Rachel Vannette is an assistant professor in the Department of Entomology and Nematology at UC Davis. Vannette is the lead author of the research paper “Dispersal enhances beta diversity in nectar microbes.”

“We chose Sticky Monkeyflower because it was abundant at the field site we were working at,” Vannette said. “It produces a lot of floral nectar, which is a handy trait for the experiment we were looking at. We know that it has a lot of microbes in its nectar, which was probably its most important trait.”

Tadashi Fukami, an associate professor in the Department of Biology at Stanford University, is the senior author of the research paper.

“As a bird pollinator, Mimulus aurantiacus tends to produce much more nectar than other flowers,” Fukami said.

Calypte anna, commonly called Anna’s hummingbird, is the main pollinator of the Sticky Monkeyflower and the primary vehicle for yeast and bacteria transfer between plants. The birds are attracted to the sweet nectar in the flowers. Both microbes and pollen stick to the hummingbirds’ bills as the birds visit plants during the day.

Jonathan Eisen is a professor at the UC Davis Genome Center who was not involved with the research in Ecology Letters. His work involves the evolutionary process of microbial communities with a wholesome focus on science communication.

“If you want to calculate a diversity metric, most of those metrics relate to either presence and absence of species, or some measure of their relatedness among the organisms in the sample,” Eisen said. “One is called taxonomic diversity — that’s presence and absence of species. If you look at the relatedness of organisms — that’s called phylogenetic diversity, [or] evolutionary diversity. You can also try to do things like look at traits — or functional diversity.”

Each Mimulus flower contains millions of bacteria and yeast feeding from the sugary nectar. The flower serves as a home habitat for microbes, which adjust the chemistry of the nectar as they grow and develop. Flowers rich in Acetobacter bacteria will convert ethanol into acetic acid, creating a nectar environment which is unsuitable to certain other microbes and distasteful to unsuspecting pollinators.

“For each of these [diversity metrics], you can measure them within a community — that’s ‘alpha diversity’; or compare them between communities — and that’s usually called ‘beta diversity’,” Eisen said.

Measuring the beta diversity between the microbial communities living within Sticky Monkeyflowers meant comparing the rates of pollinator dispersal and counting the varied bacteria and yeast communities found in the flowers.

The fieldwork was completed at Jasper Ridge Biological Preserve in the Santa Cruz Mountains. Sticky Monkeyflower tends to stay close to the sunny paths of the hiking trails. Collecting flowers could therefore be done without straying too far into the trees.

“We wanted to minimize damaging the chaparral and forest habitats,” Fukami said.

Mimulus flowers were separated into three experimental groups. Samples were collected during the spring and early summer, while the flowers were blooming.

“We went to the field and blocked dispersal of microbes using simple methods,” Fukami said. “Basically, we did two things to different flowers. We bagged some flowers. By doing that, we denied access to flowers by pollinators. We [also] had a cage with some mesh much coarser than the bag, so small insects would have no trouble accessing the flowers. The purpose was to not let hummingbirds access the flowers, but let the bees in.”

Another group of flowers was left uncovered. All pollinators could access this third group of flowers as desired. Though a prominent ecological theory suggests that the more dispersal occurring in a community, the lower the diversity of organisms will be, Vannette’s observations provide evidence suggesting the opposite.

“When you compare the microbial communities among flowers, we thought that those flowers that had more dispersal should look more similar to each other, right?” Vannette said. “Because they had been connected, and the microbes have presumably been moved from one to the other, so they should look the same. But that’s actually not what we found.”

Vannette’s research may have found different results from previous ecology experiments due to the implementation of a thoughtful, natural design. Birds that pollinate the Sticky Monkeyflower and transfer bacteria and yeast between flowers were observed performing these duties realistically in Vannette’s experiment. Previous experiments used flowers in laboratories, with pipettes serving as the pollinator.

“We found that those flowers that had been visited more often, or that were available to be visited more often, actually had the most divergent communities compared to those that had limited dispersal,” Vannette said. “From some of our previous work, we think that one mechanism for that could be priority effects, or basically resource preemption, by some of the microbes which get there first.”

Improvements in molecular biology have allowed scientists to compile genetic libraries of microbes to begin creating bacterial field guides, such as the ones wildlife specialists use to identify birds.

“Historically, most studies of the principles of ecology have come from studying macroorganisms that you can easily observe without a microscope or without some alternative technology,” Eisen said. “[Vannette and Fukami are] taking advantage of DNA technology to include microbes as part of an ecological study.”

Vannette and Fukami were assisted in their work by an undergraduate field study course at Stanford University. Students helped collect hundreds of Mimulus flowers to be analyzed by Vannette’s team on group hikes to Jasper Ridge. Over a hundred students meaningfully aided the research team with their time and labor.

“Our results are robust and not dependent on a few flowers,” Fukami said. “The large dataset makes this study strong.”

The new research from Vannette’s group improves the collective understanding of the ecological community. However, a concrete, long-standing theory of how pollinator dispersal affects microbial community diversity requires more fine-tuning in light of improved experimental techniques.

“We know very little about the dispersal of microbes across landscapes,” Eisen said. “This, to me, is a really interesting system for asking and answering questions about how communities of microbes get dispersed in a natural landscape.”

Written by: George Ugartemendia — science@theaggie.org